Field of the Invention
This invention relates to a method of controlling the direction of emission of radio waves from a radio transmitter which transmits radio waves (a beam) from a base-station antenna in the direction of a mobile station while providing the radio waves with directivity, to a radio transmitter and radio receiver for implementing this method, and, more particularly, to a method and apparatus for measuring the direction of a mobile station and transmitting a beam in this direction.
In mobile radio communication systems, a base station cannot use a fixed directional pattern for communication with a mobile station; the base station performs communication using a non-directional antenna. However, since transmission by a non-directional antenna emits radio waves in directions in which the target mobile station does not exist, not only is power efficiency poor but the fact that mobile stations other than the target mobile station are subjected to radio interference degrades communication quality. As a consequence of such interference, a frequency that has been used in communication with a certain mobile station can be re-utilized only at a location far enough away for the radio waves to be attenuated sufficiently. This results in inefficient utilization of frequencies. A method of improving frequency utilization efficiency by establishing sectors (sector zones) and using a sector antenna is known in the art (see Okumura, Shinji, xe2x80x9cFoundations of Mobile Communicationsxe2x80x9d, Electronic Information Communication Institute, 1986). FIG. 21 is a diagram useful in describing a sector antenna. As shown in (a) of FIG. 21, the 360xc2x0 perimeter of a base station is equally divided to split a cell into a plurality of sectors SC. A sector antenna is an antenna that is allocated to each sector SC. There is no directivity within a sector. The technique for establishing sectors merely reduces the 360xc2x0 range of non-directivity to a narrower range of non-directivity such as 120xc2x0. The narrower sector is still susceptible to interference from other users or subjects other users to interference. Such interference is the main cause of a decline in channel capacity and transmission quality.
For this reason, it is necessary to measure the position of the mobile station successively in order to transmit radio waves with a narrow directivity in the direction of the mobile station, as illustrated in (b) of FIG. 21. The position of a mobile station can be determined if the mobile station is set up for a position measurement system such as the GPS (Global Positioning System). However, not all mobile stations are necessarily capable of utilizing a position measurement system and therefore the method of relying upon a position measurement system is not appropriate. A proposed method that does not employ a position measurement system is to find the direction of arrival of uplink radio waves by subjecting a received signal to signal processing and then transmit radio waves in this direction. For example, see L. C. Godara, xe2x80x9cApplication of antenna arrays to mobile communications, PT. II; Beamforming and direction-of-arrival considerations,xe2x80x9d Proc. IEEE, vol. 85, no. 8, pp. 1195-1245, August 1977.
However, the proposed method of measuring the direction of arrival of uplink radio waves by the signal processing of a received signal involves a heavy processing load, such as a requirement to calculate eigenvalues, and necessitates a complicated apparatus.
Accordingly, an object of the present invention is to measure the direction of a mobile station through a simple arrangement.
Another object of the present invention is to find the direction of a mobile station (the direction in which a radio base station should point its radio waves) through a simple arrangement, and emit the radio waves in this direction upon providing the radio waves with directivity.
A further object of the present invention is to emit radio waves in the measured direction of a mobile station using an array antenna.
(a) Measurement of receiver direction
In order to measure the direction of a receiver according to the present invention, (1) first and second signals that have been spread by mutually orthogonal spreading codes are transmitted from antennas of a base station that are disposed at different positions, or the same signal is time-shared and transmitted as first and second signals from antennas of a base station that are disposed at different positions; (2) the first and second signals that have been transmitted from the respective antennas are received by a receiver and the phase difference between these signals is found; and (3) the direction of the receiver as seen from the transmitter of the base station is calculated based upon the phase difference. In this case, in a multipath environment, the path among multipaths along which a signal will arrive earliest is found and the phase difference between the first and second signals that arrive via this path is calculated. If we let D represent the interval between the two antennas, xcex the wavelength of the radio waves, xcex8 the direction of the mobile station and xcfx86 the phase difference between the first and second received signals, these will be related as follows: xcfx86=2(xcfx80/xcex)xc2x7Dxc2x7sin xcex8. Accordingly, if xcfx86 is measured, then "THgr" can be found from the above equation. Thus, in accordance with the method of measuring the direction of a receiver according to the present invention, the receiver direction can be measured in a simple manner. Further, according to the present invention, since the direction of a mobile station is measured using a signal that arrives earliest via multipaths, there is no influence from radio waves that arrive owing to reflection or scattering. This makes it possible to measure direction accurately.
(b) Controlling direction of radio-wave emission
In order to emit radio waves with directivity in the direction of a receiver according to the present invention, (1) the direction xcex8 of a receiver is measured by the above-described method of measuring receiver direction; (2) the direction xcex8 is fed back from the receiver to the base station; and (3) radio waves are emitted from the transmitter of the base station in the direction of the receiver on the basis of the receiver direction using a directional antenna, whereby data is transmitted.
According to another aspect of the present invention, (1) the interval between two antennas that emit first and second signals is made equal to the interval between antenna elements of an equally spaced linear array antenna for data transmission; (2) a receiver receives the first and second signals transmitted from the antennas and finds a phase difference xcfx86 between the received signals; (3) the phase difference xcfx86 is fed back from the receiver to a base station; (4) a transmitter of the base station emits radio waves in the direction of the receiver upon providing the radio waves with directivity by applying the phase difference successively in steps of xcfx86 to a data signal that is input to each of the antenna elements of the equally spaced linear array antenna. If this arrangement is adopted, the phase difference need only be detected and fed back, making it unnecessary to calculate the receiver direction xcex8.
According to another aspect of the present invention, (1) first and second reference signals that have been spread by mutually orthogonal spreading codes are generated; (2) a prescribed phase difference is successively applied to the first reference signal, the resultant signals are input to each of the antenna elements of an equally spaced linear array antenna, the phase difference is successively applied to the second reference signal and the resultant signals are input to each of the antenna elements of the equally spaced linear array antenna in such a manner that a phase reference point of the first and second reference signals shifts by an amount equivalent to the interval between antenna elements of the equally spaced linear array antenna; (3) a receiver receives the first and second reference signals sent from a transmitter of a base station and finds a phase difference xcfx861 between the received first and second reference signals; (4) the phase difference xcfx861 is fed back from the receiver to the base station; and (5) the transmitter of the base station emits radio waves in the direction of the receiver upon providing the radio waves with directivity by applying the phase difference xcfx861 successively to a data signal that is input to each of the antenna elements of the equally spaced linear array antenna.
In order to so arrange it that the phase reference point of the first and second reference signals shifts by an amount equivalent to the interval between antenna elements of the equally spaced linear array antenna, (1) a prescribed phase difference is successively applied to the first reference signal and the resultant signals are input to the equally spaced linear array antenna from a first antenna element thereof to an (nxe2x88x921)th antenna element thereof in succession; and (2) the prescribed phase difference is successively applied to the second reference signal and the resultant signals are input to the equally spaced linear array antenna from a second antenna element thereof to an nth antenna element thereof in succession.
If the above-described arrangement is adopted, reference signals for direction measurement can be emitted from an equally spaced linear array antenna for data transmission. This makes it unnecessary to separately provide an antenna for direction measurement. Further, the same signal can be generated in time-shared fashion and input to an equally spaced linear array antenna as the first and second reference signals, thereby making it possible to simplify the construction of the apparatus.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.